1. Field of the Invention
This invention relates to electronic ignition systems or the like and more particularly to a noise blanker circuit for use therein to inhibit false signals from being generated due to system noise transients.
2. Description of the Prior Art
Electronic ignition systems which are operated in timed relationship to an automobile engine rpm for controlling a charge and discharge of an ignition coil to generate spark to operate the engine are well known in the art. Some contemporary ignition systems utilize adaptive dwell to control the power dissipated in the high energy ignition coil according to engine rpm. For example, FIG. 1 illustrates an adaptive dwell ignition system wherein the dwell current in the ignition coil is initiated anywhere from 25%-90% after the initiation of a firing cycle depending upon the engine rpm and the current ramp time of the ignition coil as is generally understood. Because the ignition coil is a high energy device the current through the coil ramps very fast since the RL time constant of the coil is very low. Hence, voltage noise transients can be produced across the windings of the coil that may be equal to 2,000 volts or more. It has been observed that these voltage transients can be both inductively and capacitively coupled to the inputs of the ignition system and have a deleterious effect on the operation thereof. For example, in normal operation of the ignition system the coil dwell current is produced some time after the first 25% time period of the firing cycle and is allowed to ramp up through the coil to a maximum current value at which time the current is limited. At the end of the firing cycle, in response to the initiation of the next succeeding firing cycle, a spark command signal is generated by the ignition system to cause the coil to be discharged which creates the spark necessary to operate the engine. It has been observed that due to the low RL time constant to the coil that the dwell current therethrough produces voltage transients across the windings of the coil to cause the ignition system to generate what appears to be a valid spark command signal. This can cause a misspark or no spark to occur at the correct time during the firing cycle period which may damage the engine. These voltage transients normally decay to a sufficiently low level within 11/2 milliseconds after initiation of the coil current.
Thus, there is a need for a blanking circuit to be utilized in combination with an electronic ignition system or the like that requires a minimal number of components for blanking out a portion of each periodic signal input supplied thereto for a predetermined time interval after initiation of each period in order to prevent false output signals from occurring at the output of the system due to noise transients.